Since the announcement of the first commercial installation of Octafining in Japan in June, 1958, this process has been widely installed for the supply of p-xylene. See "Advances in Petroleum Chemistry and Refining" volume 4 page 433 (Interscience Publishers, New York 1961). That demand for p-xylene has increased at remarkable rates, particularly because of the demand for terephthalic acid to be used in the manufacture of polyesters.
Typically, p-xylene is derived from mixtures of C.sub.8 aromatics separated from such raw materials as petroleum naphthas, particularly reformates, usually by selective solvent extraction. The C.sub.8 aromatics in such mixtures and their properties are:
______________________________________ Freezing Boiling Point .degree. F. Point .degree. F. ______________________________________ Ethylbenzene -139.0 277.1 P-xylene 55.9 281.0 M-xylene -54.2 282.4 O-xylene -13.3 292.0 ______________________________________
Principal sources are catalytically reformed naphthas and pyrolysis distillates. The C.sub.8 aromatic fractions from these sources vary quite widely in composition but will usually be in the range 10 to 32 wt. % ethylbenzene with the balance, xylenes, being divided approximately 50 wt. % meta, and 25 wt. % each of para and ortho.
Individual isomer products may be separated from the naturally occurring mixtures by appropriate physical methods. Ethylbenzene may be separated by fractional distillation although this is a costly operation. Ortho xylene may be separated by fractional distillation and is so produced commercially. Para xylene is separated from the mixed isomers by fractional crystallization.
As commercial use of para and ortho xylene has increased there has been interest in isomerizing the other C.sub.8 aromatics toward an equilibrium mix and thus increasing yields of the desired xylenes. At present, several xylene isomerization processes are available and in commercial use.
The isomerization process operates in conjunction with the product xylene or xylenes separation process. A virgin C.sub.8 aromatics mixture is fed to such a processing combination in which the residual isomers emerging from the product separation steps are then charged to the isomerizer unit and the effluent isomerizate C.sub.8 aromatics are recycled to the product separation steps. The composition of isomerizer feed is then a function of the virgin C.sub.8 aromatic feed, the product separation unit performance, and the isomerizer performance.
It will be apparent that separation techniques for recovery of one or more xylene isomers will not have material effect on the ethylbenzene introduced with charge to the recovery/isomerization "loop". That compound, normally present in eight carbon atom aromatic fractions, will accumulate in the loop unless excluded from the charge or converted by some reaction in the loop to products which are separable from xylenes by means tolerable in the loop. Ethylbenzene can be separated from the xylenes of boiling point near that of ethylbenzene by extremely expensive "superfractionation". This capital and operating expense cannot be tolerated in the loop where the high recycle rate would require an extremely large distillation unit for the purpose. It is a usual adjunct of low pressure, low temperature isomerization as a charge preparation facility in which ethylbenzene is separated from the virgin C.sub.8 aromatic fraction before introduction to the loop.
Other isomerization processes operate at higher pressure and temperature, usually under hydrogen pressure in the presence of catalysts which convert ethylbenzene to products readily separated by relatively simple distillation in the loop, which distillation is needed in any event to separate by-products of xylene isomerization from the recycle stream. For example, the Octafining catalyst of platinum on a silica-alumina composite exhibits the dual functions of hydrogenation/dehydrogenation and isomerization.
In Octafining, ethylbenzene reacts through ethyl cyclohexane to dimethyl cyclohexanes which in turn equilibrate to xylenes. Competing reactions are disproportionation of ethylbenzene to benzene and diethylbenzene, hydrocracking of ethylbenzene to ethylene and benzene and hydrocracking of the alkyl cyclohexanes.
The rate of ethylbenzene approach to equilibrium concentration in a C.sub.8 aromatic mixture is related to effective contact time. Hydrogen partial pressure has a very significant effect on ethylbenzene approach to equilibrium. Temperature change within the range of Octafining conditions (830.degree. to 900.degree. F.) has but a very small effect on ethylbenzene approach to equilibrium.
Concurrent loss of ethylbenzene to other molecular weight products relates to percent approach to equilibrium. Products formed from ethylbenzene include C.sub.6.sup.+ naphthenes, benzene from cracking, benzene and C.sub.10 aromatics from disproportionation, and total loss to other than C.sub.8 molecular weight. C.sub.5 and lighter hydrocarbon by-products are also formed.
The three xylenes isomerize much more selectively than the reaction of ethylbenzene, but they do exhibit different rates of isomerization and hence, with different feed composition situations the rates of approach to equilibrium vary considerably.
Loss of xylenes to other molecular weight products varies with contact time. By-products include naphthenes, toluene, C.sub.9 aromatics and C.sub.5 and lighter hydrocracking products.
Ethylbenzene has been found responsible for a relatively rapid decline in catalyst activity and this effect is proportional to its concentration in a C.sub.8 aromatic feed mixture. It has been possible then to relate catalyst stability (or loss in activity) to feed composition (ethylbenzene content and hydrogen recycle ratio) so that for any C.sub.8 aromatic feed, desired xylene products can be made with a selected suitably long catalyst use cycle.
A different approach to conversion of ethylbenzene is described in Morrison U.S. Pat. No. 3,856,872, dated Dec. 24, 1974. Over an active acid catalyst typified by zeolite ZSM-5 ethylbenzene disproportionates to benzene and diethylbenzene which are readily separated from xylenes by the distillation equipment needed in the loop to remove by-products. It is recognized that rate of disproportionation of ethylbenzene is related to the rate of conversion of xylenes to other compounds, e.g. by disproportionation. See also Burress U.S. Pat. No. 3,856,873 which also describes reaction of C.sub.8 aromatics over ZSM-5 and shows effects of various temperatures up to 950.degree. F. in the absence of metal co-catalyst and in the absence of hydrogen.
In the known processes for accepting ethylbenzene to the loop, conversion of that compound is constrained by the need to hold conversion of xylenes to other compounds to an acceptable level. Thus, although the Morrison technique provides significant advantages over Octafining in this respect, operating conditions are still selected to balance the advantages of ethylbenzene conversion against the disadvantages of xylene loss by disproportionation and the like.
A further advance in the art is described in copending applications of the present applicants directed to various techniques for reducing acid activity of zeolite ZSM-5 catalyst and use of such low activity catalysts for xylene isomerization concurrently with ethylbenzene conversion at temperatures upwards of 800.degree. F. One such copending application is Ser. No. 912,681, filed June 5, 1978, now U.S. Pat. No. 4,163,028 granted July 31, 1979 which discloses xylene isomerization and ethylbenzene conversion at high temperature with ZSM-5 of very high silica/alumina ratio whereby the acid activity is reduced.
The inventions of those copending applications and the present invention are predicated on discovery of combinations of catalyst and operating conditions which decouples ethylbenzene conversion from xylene loss in a xylene isomerization reaction, thus permitting feed of C.sub.8 fractions which contain ethylbenzene without sacrifice of xylenes to conditions which will promote adequate conversion of ethylbenzene.